Effects of Si and Fe micro-additions on the aging response of a dilute Al-0.08Zr-0.08Hf-0.045Er at.% alloy

The precipitation behavior of an Al-0.08Zr-0.08Hf-0.045Er at.% alloy with micro-additions of Si and/or Fe was investigated using microhardness and electrical conductivity measurements in conjunction with scanning electron microscopy and atom-probe tomography. Hardening is achieved through the formation of a high number density (~10²³ m⁻³) of coarsening-resistant, nanoscale L1₂ trialuminide precipitates containing Zr, Hf, Er, and Si. Simultaneous additions of 300 at. ppm Si and 400 at. ppm Fe produce an alloy with the fastest precipitation kinetics and highest microhardness after homogenization at 640 °C for 24 h followed by 90 days aging at 350 °C, due to: (i) scavenging of Er by Fe in the form of primary precipitates, thus reducing Er-stimulated precipitation of coarse Zr- and Hf-rich precipitates during homogenization; and (ii) the accelerating effects of Si on the precipitation kinetics of the nanometric L1₂ trialuminide. Removal of the homogenization step results in accelerated precipitation kinetics during aging due to an increased supersaturation of L1₂-forming elements, Zr, Hf, and Er. During isothermal aging of a non-homogenized Al-0.08Zr-0.08Hf-0.045Er-0.03Si-0.04Fe (at.%) alloy at 400 °C, a peak microhardness of 500 MPa is maintained for up to 90 days. Atom-probe tomography displays a high number density of nanometric L1₂ precipitates with an Er-rich core and homogeneously distributed Zr and Hf, with Hf concentrations ~1.5 times higher at the matrix/nanoprecipitate heterophase interface than in the core (~5 vs. ~3.5 at.%). The presence of Hf in the nanoprecipitates does not, however, affect their precipitation kinetics or coarsening resistance.